The U.S. set a record for the most tornadoes within a month with April's deadly storms.
The final report for the month shows 753 twisters across the country, including a super outbreak on April 25-28 that killed more than 300 people in the South and Midwest.
While final death statistics are still being studied, the toll on April 27, being called the Dixie Outbreak, set a one-day record for tornado deaths since 1950. It topped the 310 deaths on April 3, 1974. Current estimates for the day range from 313 to 317 and could go higher, according to federal Storm Prediction Center data.
Follow-up studies that eliminated duplicate reports cut April's tornado total down from the preliminary count of 875 that generated widespread publicity. But the storm center says that still tops the former monthly record of 543 tornadoes in May 2003.
Atmospheric scientists noted that April had an active weather pattern across the 48 contiguous states, with strong storms moving through the center of the country, tapping into moisture from the Gulf of Mexico as they matured across the mid-Mississippi Valley. Contributing to the thrashing were the La Nina conditions in the Pacific Ocean, unusually warm ocean temperatures in the Gulf of Mexico and the increase of moisture in the atmosphere caused by the warming climate.
Thomas Karl, head of the National Climatic Data Center, has cautioned against focusing on any single cause for the unusual chain of events, saying "clearly these things interconnect."
The tornado death toll for the year so far is 546, including 364 in April and another 177 in May, generally the two busiest months for twisters. That total ranks this year fourth overall in tornado fatalities, still well behind the record 794 deaths in 1925.
In addition to the high tolls in April and May, three people were killed by tornadoes in June and there was one death each in February and March.